This invention relates to a method of controlling the size of magnetic nanoparticles.
For many applications, it is highly desirable to have magnetic nanoparticles whose sizes are all within a very limited range (substantially monodisperse). For some applications, it is desirable to employ nanoparticles that are sufficiently small that they can support only a single magnetic domain state. It would be very convenient to be able to manufacture such particles using chemical reactions in solution using a continuous reactor system.
Size control is very important in the synthesis of nanoparticles, and several methods have been developed to provide size control. These methods generally rely upon controlling the kinetics of the reaction to yield particles of a desired size. Examples include high-temperature nucleation to produce a number of stable nuclei, followed by low-temperature growth, where very few new nuclei are produced but materials is deposited on the previously formed nuclei. Deposition of material onto a fixed number of nuclei where additional nuclei are not being continuously formed can be used to produce particles that are relatively monodisperse in size.
However, because the sizes of the particles are controlled by reaction kinetics, such reactions are difficult to replicate in reactors with different heat and mass transport properties. This makes the scale-up of such reaction very challenging when using different sizes of reactor and different methods of temperature control or stirring. The ability to control the size of nanoparticles without relying on the heating profile or agitation level of a reaction would be highly desirable.
One example of a conventional approach is reported in Murray et al. (“Chemical Synthesis of Monodisperse and Magnetic Alloy Nanocrystal Containing Thin Films” U.S. Pat. No. 6,302,940). This reports a process based on a combination of reduction of metal salt and decomposition of neutral organometallic precursor for the formation of magnetic alloy nanoparticles. They report in situ reduction of Pt(acac)2 (acac=acetylactonate) by long chain diol and decomposition of Fe(CO)5 at a high temperature (260-300° C.) solution phase to yield high quality nanoparticles. The reduction of the metal salt by diol and the decomposition of the organometallic compound do not occur until the temperature is greater than 180° C. The particles are protected from agglomeration by a combination of long chain carboxylic acid, such as oleic acid, and long chain primary amine, such as oleyl amine. Long aliphatic chains of oleic acid present a significant steric barrier for strong interactions between the particles, and magnetic exchange coupling between the particles is eliminated completely by the physical separation induced. This stabilization is so effective that the particles can be handled easily either in solution phase or as a solid form under air. The particles can be easily dispersed in alkane and chlorinated solvents and purified by precipitation through the addition of alcohol. Their method of forming magnetic alloy nanoparticles includes forming a metal salt solution with a reducing agent and stabilizing ligands, introducing an organometallic compound into the metal salt solution to form a mixture, heating the mixtures, and adding a flocculent to cause the magnetic alloy nanoparticles to precipitate out of the mixture without permanent agglomeration.
Another example of controlling the size of nanoparticles using kinetic control is described in D. L. Huber, E. K. Venturini, J. E. Martin, P. P. Provencio, and R. J. Patel, “Synthesis of highly magnetic iron nanoparticles suitable for field structuring using a β-diketone surfactant,” Journal of Magnetism and Magnetic Materials, vol. 278 (2004) pp. 311-316. In this work, the size of nanoparticles is controlled kinetically by varying the amount of iron pentacarbonyl precursor added to a solution of pentanedione in dioctyl ether over a fixed reaction period (one hour) followed by continued reaction time at elevated temperature after cessation of the addition of the iron precursor. Iron nanoparticles of 3 nm and 6 nm diameter were grown by quadrupling the amount of added iron. This is a kinetically controlled process that does not lend itself readily to implementation in large-scale production using a continuous reactor.